The present invention relates generally to a hinge fitting for seats and, more particularly, to seats in which a backrest member is angularly adjustable with reference to a seat member. The invention is particularly but not exclusively suitable for use on seats of automotive vehicles.
It has been proposed in the prior art to connect a backrest mounting member and a seat mounting member respectively to a backrest part and a seat part of a seat. A ring gear having inwardly directed teeth is fixedly mounted on the seat mounting member, and a spur gear having outwardly directed teeth is fixedly mounted on the backrest mounting member and matingly engages the ring gear. The spur gear has an outer diameter smaller by the height of at least one tooth than the root diameter of the ring gear. The spur gear is mounted on a cam of a rotatable pin. A self-locking type of action between the spur gear and the ring gear is obtained by selecting the eccentric or radial offset distance of the cam to be approximately equal to the difference between the root diameter of the ring gear and the outer diameter of the spur gear.
However, this known proposal is disadvantageous in that the backrest mounting member does not move in a circular path of constant radius relative to the seat mounting member. The backrest mounting member is fixedly connected to the spur gear and, since the spur gear traverses an eccentric path of unequal radius, the backrest mounting member is forced to travel along such an eccentric path. This means that the angular speed of the backrest mounting member during its adjustment is non-uniform and, in fact, this angular speed varies in dependence upon the particular angular position of the spur gear relative to the ring gear.
Furthermore, it is frequently desirable to use spiral springs in order to hold the turning torque of the adjustment hand wheel used in rotating the cam to within acceptable limits, particularly when heavy back seat parts are to be adjusted. However, such springs cannot be satisfactorily used when the back seat mounting member moves through an eccentric path because the spring force varies due to the fact that the lever arm of the spring changes during such eccentric movement of the backrest mounting member.
In addition, such eccentric movement of the backrest mounting member means that the turning torque to the hand wheel also varies as a function of the particular angular position of the spur gear relative to the ring gear. This is extremely undesirable for a user since it makes simple adjustment of the seat very difficult.
A further disadvantage which results from the eccentric movement of the two mounting members is that the means for turning the rotatable pin on which the cam is mounted can be mounted only on the rotatable pin itself or else only on the stationary mounting member.